As a kind of important components of all-solid-state lasers, nonlinear optical crystals have always been widely concerned both at home and abroad. After half a century's researches, a series of nonlinear optical crystals with excellent properties have been found. Typical nonlinear optical crystals used in the visible light region include KTiOPO4 (KTP), KH2PO4 (KDP), etc. Nonlinear optical crystals used in the ultraviolet region that have been industrialized include LiB3O5 (LBO), CsB3O5 (CBO), CsLiB6O10 (CLBO), BaB2O4 (BBO), etc. However, KBe2BO3F2 (KBBF) is the only available crystal that can apply in the deep-ultraviolet region below 200 nm. The application of KBBF is restricted due to its long growth period highly toxic element Be and layer growth habit, etc. Therefore, it is necessary to explore new deep-ultraviolet nonlinear optical crystals with better performances.
The inventors developed a compound of ammonium fluorooxoborate, NH4B4O6F, and a nonlinear optical crystal of NH4B4O6F in previous studies, as described in the patent application No. 201611128283.3. The main differences between the present invention and NH4B4O6F is that the NH4+ is linked to the anion group by a hydrogen bond in NH4B4O6F, while the Cs+ is linked to the anion group by an ionic bond in CsB4O6F, resulting in completely different structures and growth habits, as well as different key parameters of the growth process, crystal properties, and the like between CsB4O6F and NH4B4O6F.